When short-wavelength (e.g., millimeter-wave) high-frequency signals are transmitted from an antenna, transmission loss is increased in directly providing high-frequency signals to the antenna from a circuit chip. In response, Japanese Laid-open Patent Publication No. 2006-340317 describes a technology configured to convert high-frequency signals from a normal mode to a waveguide-tube propagation mode and subsequently provide the post mode-conversion high-frequency signals to the antenna in order to reduce the transmission loss.
A high-frequency circuit module of the well-known type will be hereinafter explained with reference to FIG. 12. FIG. 12 is a schematic cross-sectional view of the high-frequency circuit module of the well-known type. As illustrated in FIG. 12, the high-frequency circuit module 1 of the well-known type includes a hollow waveguide tube 2, a waveguide substrate 3, and a semiconductor circuit chip 4. The hollow waveguide tube 2 is mounted on the waveguide substrate 3. The waveguide substrate 3 includes a waveguide 3A for transmitting high-frequency signals. The waveguide 3A is coupled to the hollow waveguide tube 2. The semiconductor circuit chip 4 is mounted on the waveguide substrate 3.
The waveguide substrate 3 includes a dielectric plate 31, conductor layers 32a, 32b, and a plurality of conducting posts 33. The conductor layers 32a, 32b are disposed on the both sides of the dielectric plate 31. The conducting posts 33 are aligned in two rows while each low includes a plural number of conducting posts 33. The conducting posts 33 are configured to establish electrical conduction between the conductor layer 32a disposed on one side of the dielectric plate 31 and the conductor layer 32b disposed on the other side of the dielectric plate 31. The waveguide 3A is a dielectric part enclosed by the conductor layers 32a, 32b and the conductive posts 33 aligned in two rows.
The waveguide substrate 3 is supported by a support member 6.
An island-shaped metal pad 37 is disposed on the surface of the waveguide substrate 3 that the semiconductor circuit chip 4 is mounted. Specifically, the metal pad 37 is surrounded by the conductor layer 32a through a gap 37a. The metal pad 37 is connected to a signal line of the semiconductor circuit chip 4 in an upstream position within the waveguide 3A.
Further, a metal-pad conducting post 33d is disposed in the waveguide substrate 3. FIG. 13 is a cross-sectional view of the high-frequency circuit module sectioned along a line A-A′ in FIG. 12. As illustrated in FIG. 13, an underfiller 43 is filled in the clearance between the semiconductor circuit chip 4 and the waveguide substrate 3. Accordingly, the semiconductor circuit chip 4 is mounted on the waveguide substrate 3 by flip-chip bonding. Further, a signal line 41 of the semiconductor circuit chip 4 is connected to the metal pad 37 through a metal bump 41b. Meanwhile, the metal pad 37 is connected to the conductor layer 32b through the metal-pad conducting post 33d. High-frequency signals from the signal line 41 of the semiconductor circuit chip 4 are converted from the normal mode to the propagation mode for propagating the waveguide 3A (hereinafter referred to as the waveguide-3A propagation mode) through the metal-pad conducting post 33d. 
In the high-frequency circuit module 1 of the well-known type, the gap 37a and the metal-pad conducting post 33 are formed in different processing steps. Therefore, positional displacement may occur between the gap 37a and the metal-pad conducting post 33d in the manufacturing processing of the high-frequency circuit module 1. The positional displacement produces a drawback of reduction in efficiency of converting high-frequency signals, transmitted from the signal line 41 of the semiconductor circuit chip 4, from the normal mode to the waveguide-3A propagation mode